linux-kernel - Re: [PATCH] usb: Clear both buffers when clearing a control transfer TT buffer.

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Date:	Fri, 28 Jun 2013 11:56:26 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	William Gulland <wgulland@...gle.com>
cc:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	Lan Tianyu <tianyu.lan@...el.com>,
	Ming Lei <ming.lei@...onical.com>, <linux-usb@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: Clear both buffers when clearing a control transfer
 TT buffer.

On Thu, 27 Jun 2013, William Gulland wrote:

>     Control transfers have both IN and OUT (or SETUP) packets, so when
>     clearing TT buffers for a control transfer it's necessary to send
>     two HUB_CLEAR_TT_BUFFER requests to the hub.

What makes you think this is necessary?  Have you found any hardware 
that requires this?

Although the USB spec doesn't say much about Clear-TT-Buffer requests,
the text in section 11.17.1 and Figure 11-47 seems to indicate that a
second Clear-TT-Buffer shouldn't be needed.  The spec says that TT
buffers are matched based on the device address, endpoint number, and
endpoint direction -- but the direction is used only for bulk
endpoints, not for control endpoints.

In particular, a control endpoint uses only one TT buffer for both the
IN and OUT directions.  So I don't think two Clear-TT-Buffers are
needed.

Alan Stern

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